Conventionally, there is a demand for making a semiconductor substrate, which is used for forming a semiconductor device such as a power metal oxide semiconductor (MOS), thinner to reduce a manufacturing cost, and, thus, a thin-type semiconductor substrate has been manufactured.
However, a thinned semiconductor substrate of 300 (μm) or less is close to a paper or a film, and it is difficult to be subjected to handling such as a conveyance in a semiconductor manufacturing process. Thus, in order to eliminate such a problem, there is known a wafer of which an inner area of a boundary line, which is at a position offset inwardly from the outer edge of the wafer, is thinned and a material having a rigidity higher than the wafer covers an inner wall of a thick part exiting on outer side of the boundary line to increase a strength (for example, refer to Patent Document 1). Additionally, as a manufacturing method of the wafer disclosed in the Patent Document 1, there is disclosed a manufacturing method including a trench forming process of forming a trench extending along the boundary line from a front surface of the wafer to a back surface by applying an anisotropic etching to a range along the boundary line, and a removing process of removing a semiconductor exiting inside the trench until the surface of the wafer existing inside the trench reaches a bottom surface of the trench.
In the wafer disclosed in the Patent Document 1 and the manufacturing method thereof, the inner wall of the trench is formed in flat and smooth by continuing the etching to maintain the aspect ratio of the trench to 7 to 9 when forming the trench, and the trench of which a connecting part between the inner wall and the bottom surface is rounded is formed. Thereby, the wafer can be made so that the inner wall of the thick part is flat and smooth and a stress is hardly concentrated into a connecting part between the thick part and a thin part, and it is hardly destroyed and easy to handle.    Patent Document 1: Japanese Laid-Open Patent Application No. 2008-218820